Amorphous materials (for instance, glass and rubber) have order within a few atomic or molecular dimensions.

Polycrystalline materials have a high degree of order over many atomic or molecular dimensions. These ordered regions, or single-crystal regions, vary in size and orientation with respect to one another. The single-crystal regions are called grains and are separated from one another by grain boundaries.

Single-crystal materials (for example, silicon, copper, and table salt) have a very high degree of order, or regular geometric periodicity, throughout their entire volume. The advantage of a single-crystal material is that, in general, its electrical properties are superior to those of a nonsingle-crystal material, since grain boundaries tend to degrade the electrical characteristics.

A grain boundary is the interface between two grains in a polycrystalline material. Grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve strength, as described by the Hall-Petch relationship. Since grain boundaries are defects in the crystal structure they tend to decrease the electrical and thermal conductivity of the material. The high interfacial energy and relatively weak bonding in most grain boundaries often makes them preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. They are also important to many of the mechanisms of creep.